Energy and environment issues are the two strategic problems to be addressed for the sustainable development of human race, and it is getting more and more important and urgent to exploit and utilize the clean and renewable energy. Solar energy is a kind of clean, abundant, locally available and renewable energy. It is of great significance to exploit and utilize solar energy. Solar cells are one of the most effective devices for utilizing the solar energy, among which, copper-indium-gallium-sulfur-selenium (hereafter abbreviated as CIGSS) thin film solar cells have been recognized as the most promising next generation of solar cells, which represents the advantages of low-cost, high-efficient, long-term stable, superior performance under weak illumination, and desirable resistance to radiation. However, commercial mass production of CIGSS thin film solar cells has not been realized because of the complicated conventional process for preparing the light absorption layer of CIGSS thin film solar cells, leading to a low yield rate and a high production cost.
The methods used for preparing light absorption layer of CIGSS thin film solar cells can be categorized into two classes. One is the high-vacuum vapor deposition methods, such as thermal evaporation, magnetron sputtering and molecular beam epitaxy. CIGSS thin films with small area prepared by above methods possess excellent quality, and the corresponding solar cells can exhibit very high photoelectric conversion efficiencies. As disclosed by the US National Renewable Energy Laboratory (NREL), a highest efficiency of 19.5% has been achieved with a copper-indium-gallium-selenium (hereafter abbreviated as CIGS) thin film solar cell with an effective area of 0.405 cm2 prepared by the so-called three-stage co-evaporation process. However, it would be difficult to ensure the uniformity of thin films when these methods are used for the deposition of large-area thin films. Moreover, various factors such as low yield rate resulted from the complexity of those processes, high capital investment, low raw material utilization rate and low productivity, leads to a very high production cost, which prohibits the mass production of CIGSS thin film solar cells by these methods. The other type of method is a non-vacuum liquid phase methods. As compared with the conventional high vacuum vapor phase methods, substantial cost reduction can be achieved when using these non-vacuum liquid phase methods and large area thin films can be conveniently deposited. Extensive and intensive researches have been conducted in recent years as for the preparation of light absorption layer of CIGSS thin film solar cells by non-vacuum liquid phase method, which can be categorized as following:
(1) Oxide-Based Non-Vacuum Liquid Phase Method
The oxide-based non-vacuum liquid phase method comprises the following steps: (a) preparing of the liquid phase precursor of micro-particles of the oxides of copper, indium and gallium, etc., (b) coating the liquid phase precursor on substrates by various non-vacuum processes to produce precursor films, (c) reducing the precursor films under high temperature followed by selenizing them at high temperature to produce CIGS thin films. Kapur, etc. have reported an oxide-based non-vacuum liquid phase method, which was characterized in that the oxides in the liquid phase precursor were particulates with sub-micron size prepared by mechanical ball milling (See U.S. Pat. No. 6,127,202). In the method developed by Eberspacher and Pauls, the sub-micron sized complex oxides particulates which were produced by the pyrolytic decomposition of liquid drop are spayed under supersonic wave onto the substrate to obtain the precursor thin film (U.S. Pat. No. 6,268,014).
Though it is quite cost efficient to prepare light absorption layers of CIGS thin film solar cells by oxide-based non-vacuum liquid phase method, this method also exhibits great drawbacks. Firstly, it will be a waste of time and energy to reduce oxide-based precursor thin films in H2 atmosphere at high temperature. Secondly, it is very hard to reduce the precursor thin films completely because of the extremely high stability of gallium oxide, which will result in a high concentration of impurities in the targeted CIGS thin films and poor doping of gallium. Lastly, thorough selenization of copper-indium-gallium alloy thin films produced by the reduction of oxides is very hard to achieve, which is due to the reaction kinetics mechanism.
(2) Spray Pyrolysis
It is quite cost efficient to prepare CIGSS thin films by spray pyrolysis method, however, high concentration of detrimental impurities, high roughness and un-uniformity in large area thin films hindered the practical utilization of this method.
It is very hard to prepare CIGS thin films qualified for the photovoltaic devices by spray pyrolysis, and solar cells prepared by this process show extremely low photoelectric conversion efficiency, which almost excludes the industrial application of this method in CIGS thin film solar cells.
(3) Electrochemical Method
Considerable attention has been attracted to the electrochemical deposition of CIGS thin films ever since the first successful deposition of CuInSe2 thin films by electrochemical method reported by Bhattacharya (J. Electrochem. Soc. 130, 2040, 1983) in 1983. A two-step deposition process was also developed by Bhattacharya, which is characterized in that a copper-rich CIGS thin films were firstly deposited by electrochemical method, followed by adding additional In, Ga and Se, etc. to the films. Thus the final composition of the targeted thin films is modified so as to fulfill the criterion of solar cells. A CIGS thin film solar cell with a photoelectric conversion efficiency of 15.4% was fabricated employing the two-step deposition process, and it is by far the best performed one prepared by electrochemical deposition method.
Low cost, high utilization rate of raw materials and facile deposition of large area thin films are typical advantages of electrochemical deposition method. However, very large gaps existing between reduction potentials of Cu, In and Ga often bring about enrichment of copper, great difficulties in the stoichiometry control and high concentration of impurities in the produced thin films. Subsequent modification of the stoichiometry of thin films by PVD is usually necessary, which would lead to a sharp increase in production cost.
(4) Non-Oxide-Based Non-Vacuum Liquid Phase Method
Non-oxide-based non-vacuum liquid phase method was developed by Nanosolar corp. for preparing CIGS thin films (U.S. Pat. No. 7,306,823). This method comprises the following steps: firstly, preparing nanoparticles or quantum dots of copper or indium or gallium or selenium; secondly, coating the surface of nanoparticles or quantum dots with one or more layers of copper, indium, gallium, and selenium, etc. wherein the stoichiometry ratios between different elements in the coated nanoparticles are controlled by adjusting the composition and thickness of the coating layer; thirdly, dispersing the coated nanoparticles in a solvent to produce a slurry; fourthly, forming a precursor thin film from the slurry by a non-vacuum process such as printing, etc.; and lastly, short annealing the precursor film to produce the targeted CIGS thin films.
Low cost, high utilization rate of raw materials, applicability of flexible substrates and facile deposition of large area thin films can be readily achieved by this method. However, since nano-particles are used in this method, and parameters of the coated nanoparticles, such as particle size, size distribution, surface morphology and stoichiometry are very hard to be precisely controlled, thus resulting in unfavorable controllability, high complexity and poor reproducibility of the process.
In view of above, currently available methods for producing CIGSS thin films exhibits defects of one kind or another, which hampers the large-scale commercialization of CIGS thin film solar cells. Developing a novel method for producing CIGSS thin films that can overcome above defects would be a great impetus, and it would be of great significance to the industrialization of CIGS thin film solar cells.